AP Biology Unit 3 Essentials

Most chemical reactions in cells would happen far too slowly to support life — they're held up by the energy barrier called activation energy. Enzymes solve this by providing a precisely shaped active site that holds substrates in just the right position, lowering the activation energy and accelerating the reaction by millions or billions of times. Because enzymes are proteins, their function depends entirely on their shape: change the structure with heat, pH, or inhibitors, and the active site stops working. This is the structure–function rule applied to metabolism.

Photosynthesis uses light energy to push electrons uphill, fixing CO₂ into glucose and releasing O₂. Cellular respiration runs the reverse overall equation: glucose + O₂ broken down to CO₂ + H₂O, with the energy captured as ATP. Both pathways rely on a key trick — an electron transport chain that pumps H⁺ to create a gradient, which then drives ATP synthase via chemiosmosis. This is how most of the world's ATP is made. The light reactions of photosynthesis and the oxidative phosphorylation of respiration are built on the same fundamental mechanism.

Every living thing — from bacteria to redwoods to humans — uses ATP as its energy currency. Every cell builds it with the same machinery (ATP synthase), and every cell hydrolyzes it the same way to power coupled reactions. Combined with the universality of glycolysis (the oldest pathway, present in every kind of organism), ATP's universality is one of the strongest pieces of evidence that all life shares a common ancestor. It also means a drug or toxin that disrupts ATP production tends to affect every kind of cell.